In a typical wireless communication network, wireless devices, also known as wireless communication devices, mobile stations, stations (STA) and/or user equipments (UE), communicate via a Radio Access Network (RAN) to one or more core networks (CN). The RAN covers a geographical area which is divided into service areas or cell areas, with each service area or cell area being served by a radio network node such as a radio access node e.g., a Wi-Fi access point or a radio base station (RBS), which in some networks may also be denoted, for example, a “NodeB” or “eNodeB”. A service area or cell area is a geographical area where radio coverage is provided by the radio network node. The radio network node communicates over an air interface operating on radio frequencies with the wireless device within range of the radio network node.
A Universal Mobile Telecommunications System (UMTS) is a third generation (3G) telecommunication network, which evolved from the second generation (2G) Global System for Mobile Communications (GSM). The UMTS terrestrial radio access network (UTRAN) is essentially a RAN using wideband code division multiple access (WCDMA) and/or High Speed Packet Access (HSPA) for user equipments. In a forum known as the Third Generation Partnership Project (3GPP), telecommunications suppliers propose and agree upon standards for third generation networks, and investigate enhanced data rate and radio capacity. In some RANs, e.g. as in UMTS, several radio network nodes may be connected, e.g., by landlines or microwave, to a controller node, such as a radio network controller (RNC) or a base station controller (BSC), which supervises and coordinates various activities of the plural radio network nodes connected thereto. This type of connection is sometimes referred to as a backhaul connection. The RNCs and BSCs are typically connected to one or more core networks.
Specifications for the Evolved Packet System (EPS), also called a Fourth Generation (4G) network, have been completed within the 3rd Generation Partnership Project (3GPP) and this work continues in the coming 3GPP releases, for example to specify a Fifth Generation (5G) network. The EPS comprises the Evolved Universal Terrestrial Radio Access Network (E-UTRAN), also known as the Long Term Evolution (LTE) radio access network, and the Evolved Packet Core (EPC), also known as System Architecture Evolution (SAE) core network. E-UTRAN/LTE is a variant of a 3GPP radio access network wherein the radio network nodes are directly connected to the EPC core network rather than to RNCs. In general, in E-UTRAN/LTE the functions of an RNC are distributed between the radio network nodes, e.g. eNodeBs in LTE, and the core network. As such, the RAN of an EPS has an essentially “flat” architecture comprising radio network nodes connected directly to one or more core networks, i.e. they are not connected to RNCs. To compensate for that, the E-UTRAN specification defines a direct interface between the radio network nodes, this interface being denoted the X2 interface. EPS is the Evolved 3GPP Packet Switched Domain. FIG. 1 is an overview of the EPC architecture. This architecture is defined in 3GPP TS 23.401 v.13.4.0 wherein a definition of a Packet Data Network Gateway (P-GW), a Serving Gateway (S-GW), a Policy and Charging Rules Function (PCRF), a Mobility Management Entity (MME) and a wireless or mobile device (UE) is found. The LTE radio access, E-UTRAN, comprises one or more eNBs. FIG. 2 shows the overall E-UTRAN architecture and is further defined in for example 3GPP TS 36.300 v.13.1.0. The E-UTRAN comprises eNBs, providing a user plane comprising the protocol layers Packet Data Convergence Protocol (PDCP)/Radio Link Control (RLC)/Medium Access Control (MAC)/Physical layer (PHY), and a control plane comprising Radio Resource Control (RRC) protocol in addition to the user plane protocols towards the wireless device. The radio network nodes are interconnected with each other by means of the X2 interface. The radio network nodes are also connected by means of the S1 interface to the EPC, more specifically to the MME by means of an S1-MME interface and to the S-GW by means of an S1-U interface.
The S1-MME interface is used for control plane between eNodeB/E-UTRAN and MME. The main protocols used in this interface are 51 Application Protocol (S1-AP) and Stream Control Transmission Protocol (SCTP). S1AP is the application layer protocol between the radio network node and the MME and SCTP for example guarantees delivery of signaling messages between MME and the radio network node.
Time-filtering with respect to Channel State Information (CSI), also called CSI filtering, has been discussed in 3GPP for several years. An example of a time-discrete filter is where output values y[t_n] is a function of input values x[t_i], i<=n according to a formulay[t_n]=a*y[t_(n−1)]+(1−a)*x[t_n],
where a is a filtering coefficient/parameter. The t_i are discrete time instances, where i is an integer. Up to release 12 of 3GPP specifications, time-filtering with respect to CSI has not been strictly regulated leading to that different wireless devices behave differently. This has made it difficult to design proprietary coordination features since coordination potential depends on which wireless devices are involved in the coordination.
In coming Release 13 specification a radio network node or a core network node has the capability to turn off any filtering with respect to CSI which is believed to enable an improved potential for coordination features.
Although time-filtering is bad in the context of coordination, time-filtering may be good other contexts to obtain good performance. For example, time-filtering may mitigate effect of channel estimation errors or filter away fast-fading variations of the channel that cannot be followed by the link adaptation.
The radio network node or the core network node sometimes performs time-filtering of the reported CSI to filter away fast-fading variations that cannot be or is not desired to be followed by the link adaptation. This means that CSI need to be reported often enough to make time-filtering meaningful which in turn means a higher reporting cost than if filtering would have been performed in the wireless device.
The amount of time-filtering that is optimal depend on several things such as                Coordination feature, which is a set of features when multiple radio network nodes make a coordinated decision about something e.g. coordinated scheduling when e.g. 2 or more radio network nodes jointly choose who to schedule.        Speed of the wireless device        CSI reporting periodicity        Location of the wireless device, i.e. how does the radio environment look like        
The amount of filtering that is optimal may further depend on type of CSI reporting scheme. A possible future CSI reporting scheme where improvements to the Release 13 “filtering on/off capability” can be achieved is the following:
The wireless device is configured with two types of CSI feedback:                Feedback 1: The wireless device may feedback CSI as a n×p beamforming matrix F, where n is the number of transmit antennas on the radio network node and where p is the number of ports, which also is assumed to equal the number of antennas of the wireless device. Feedback 1 is herein denoted as a beamforming matrix and thus refers to a port-to-antenna mapping, which represents the precoder of the reference signals;        Feedback 2: The wireless device may further perform another CSI feedback comprising a p×r pre-coding matrix P, where r is a proposed transmission rank and where p is the number of ports, and one or more Channel Quality Indicator (CQI) values. Feedback 2 is herein denoted as a precoding matrix.        
For Feedback 1 the wireless device estimates the n×p full channel H based on a full-dimension Channel State Indicator Reference Signal (CSI-RS), where the reported beamforming matrix F consists of the p strongest right-singular vectors of H. For Feedback 2 the wireless device evaluates a p-port CSI-RS beamformed using the beamforming matrix F to find a best pre-coding matrix P from a pre-coding codebook. A transmission to the wireless device combines both the beamforming matrix and the pre-coding matrix such as that the received p×1 vector y at the wireless device is described asy=HFPx+n,                 where x is a r×1 vector of data symbols and n is a p×1 vector of received interference and noise.        
Since reporting of F is typically much more costly than reporting of P, as F is a larger matrix and therefore the number of possible different Fs is larger, thus a representation for different Fs requires more bits, it is desired to let F to capture the slow-varying part of the channel while letting P capture the faster variations. Time filtering of the CSI for channel estimate can be used to filter away the fast variations in the channel while preserving the more stable long term spatial properties of the channel. What is considered fast respectively slow will be relative to both entities, e.g., affordable CSI reporting periodicity, known by the radio network node and entities, e.g., speed of the wireless device, known by the wireless device that may lead that a time filtering of the CSI is used that is not the optimal time filtering and thereby limiting the performance of the wireless communication network.